The one-time billionaire tycoon, toppled in a 2006 coup, has overshadowed the first fortnight of Yingluck Shinawatra's premiership with a planned trip to Japan that has raised questions about his influence over the government.

Thaksin, who lives abroad to avoid a two-year jail term for corruption, has angered the opposition with his scheduled August 22-28 visit that will include a tour of areas devastated by the March 11 earthquake and tsunami.

Emboldened by his proxies' election win, Thaksin aims to send the message that he is the "de facto prime minister", said Pavin Chachavalpongpun, a Thailand expert at the Institute for Southeast Asian Studies in Singapore.

But Pavin said the one-time tycoon is "moving too fast" and the "traditional elite and military will definitely strike back".

Thailand's generals and judges have a record of intervening in politics, particularly against Thaksin, who is loved by many poor Thais but seen by the Bangkok-based elite as authoritarian and a threat to the monarchy.

Two Thaksin parties have been dissolved by the courts in the past and there have been 18 actual or attempted coups since Thailand became a constitutional monarchy in 1932.

Thaksin's removal by royalist generals heralded five years of political crises both in the Thai parliament and on the streets, where his foes and supporters have held crippling rival protests.

This culminated in rallies by "Red Shirts" loyal to Thaksin last year, in which more than 90 people died in clashes between the army and demonstrators.

Thai political scientist Thitinan Pongsudhirak at Chulalongkorn University said Thaksin's rush back into the limelight is "provocative and not smart if he wants his sister to have a chance".

Serving a full four-year term in Thailand is the exception rather than the rule -- Thaksin is the only elected premier to have done so -- and Thitinan said "powerful adversaries" could "keep his party from running the country".

Legal pressures are already growing against the government of Yingluck, a political novice who is described by her brother as his "clone" and has raised the idea of an amnesty for convicted politicians.

Just days after the election, the Democrats launched a legal bid to have her Puea Thai party disbanded on the grounds that banned politicians such as Thaksin were involved in its campaign.

And the elite-backed opposition Democrat Party last week filed a complaint to police alleging Foreign Minister Surapong Tovichakchaikul illegally assisted the fugitive Thaksin.

While Yingluck's government denied making a specific visa request for Thaksin, Tokyo said Thailand had asked it to allow entry to the former leader, making an exception to its normal entry rules concerning criminal convictions.

Yingluck's apparent willingness to help her brother go to Japan has given the Democrats "cannon fodder" to attack the government, said Paul Chambers, senior research fellow at Payap University in northern Thailand.

The government also plans to amend the constitution drawn up by the post-coup junta in 2007, a move which Chambers said aims to "exonerate" Thaksin from previous charges, paving the way for his return to Thailand.

"If massive changes are made and Thaksin does return, such alterations will occur amidst rising anti-Thaksin demonstrations and fury within the ranks of the anti-Thaksin-led military," he said.

Overt efforts to help Thaksin could lead to pressure for the courts to investigate allegations of impropriety by the Yingluck government and possibly trigger its removal by the judiciary, Chambers added.

The previous Democrat-led government accused the fugitive of bankrolling the 2010 Red Shirt rallies and inciting unrest, and a Thai court last year approved an arrest warrant for him on terrorism charges.